Does Flexing Build Muscle?
Yes. Flexing is a great way to build muscle but it's not a matter of just standing in front of a mirror and posing. I'm sorry to shatter your dreams, but you won't win get those 19 inch biceps just by squeezing them. However, incorporating flexing into a solid workout program can definitely help you take your weight training to a new level.
3 Ways Flexing Can Build Muscle
There are a number of ways flexing can help you to build muscle. First, by flexing, you can quickly learn how to control your muscles and get a feel for when you are truly squeezing your muscles to the max. This is important because during every exercise, there is a specific part of the motion where you can transfer the weight you are using to your tendons, ligaments and joints -- something you definitely don't want to do.
For example, when you are performing preachers curls, at the top of the range of motion, you'll notice your biceps relax and the weight is transferred to your tendons and ligaments. However, if you flex and really squeeze your biceps at this stage in the exercise, you never give your bicep a chance to rest and you squeeze out that extra bit of exhaustion. This is huge if you want to build muscle fast.
Secondly, along the same line, if you learn to control your muscles and have a feel for when you are really using them, you'll be able to identify if and when you are cheating. For example, a compound exercise like the bench press uses a number of different muscles (shoulders, chest, triceps) and it's easy to let those other muscle groups pick up some of the slack when your chest is getting tired. However, if you've really developed a feeling for your muscles by flexing, you'll know when that is happening and in turn, really focus on your chest and flex the pecs throughout the entire range of motion.
Lastly, flexing is a good way to really exhaust your muscles after a taxing set. When you're completely drained and don't think you can do another rep, get in front of the mirror and flex that muscle group as hard as you can. This is a good way to keep that muscle activated and under stress - a key to building muscle quickly. By flexing, you are going to squeeze out every last drop of energy that muscle has and force it to repair itself, bigger and better. This was something Arnold Schwartzenegger constantly preached and something you constantly see him do in Pumping Iron.